1. Field of the Invention
This invention relates to a safety interlock system for an apparatus which generates either electron radiation or X-ray radiation. Such apparatuses are used e.g. for the medical treatment of patients.
2. Description of the Prior Art
It is known in the art of radiation systems to switch-off the radiation beam by utilizing an ionization chamber to which the radiation is applied, as soon as a previously determined dosage of radiation has been reached. U.S. Pat. No. 4,347,547 describes such a radiation system in which an accelerator emits electron pulses which are directed to a target for the generation of X-ray pulses. The ionization chamber is exposed to the X-ray pulses for measuring their intensity distribution. A discriminator is connected to the ionization chamber for detecting intensity inhomogeneities in the X-ray pulses. If the energy of the X-ray radiation is not between a predetermined maximum value and a predetermined minimum value, a switch is operated by the discriminator and switches off the accelerator by inhibiting the power supply of the accelerator. Simultaneously, there may also be a safety interlock of the high voltage supply to the accelerator, an RF voltage from a high frequency (HF) source for accelerating the electrons in the accelerator and/or the injection of electrons into a waveguide of the accelerator.
U.S. Pat. No. 4,342,060 discloses another safety interlock system for a linear accelerator. A measuring device determines the level of the particle beam pulses emitted by the accelerator through a target which is exposed to the particle beam pulses. A discriminator determines whether the level of the particle pulses is higher than a predetermined value. If this is the case then a switch is operated which switches off the power supply of the accelerator, the RF signals of a HF power source and/or the emission of electrons of an electron gun of the accelerator.
There are known systems which are able to generate either electron radiation or X-ray radiation. In the case of electron radiation, a scattering foil and a dose chamber for measuring the electron radiation are arranged at an exit window of the accelerator in the trajectory of the emitted electron beam. In case of X-ray radiation, a target, flattening filters for flattening the X-ray beam and a dose chamber for measuring the X-ray radiation are arranged at the exit of the accelerator in the trajectory of the electron beam and the particles emitted by the accelerator have high intensity so that they can generate enough bremsstrahlung for the generation of the X-rays. Such a system is described e.g. in U.S. Pat. No. 4,627,089 or in a publication "A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy", U.S. Department of Health and Human Services, Rockville, MD, December 1981. Such systems have been used e.g. for the medical treatment of patients with electron radiation or with X-ray radiation.
If a failure occurs during the operation of such a system and the particles having high intensity, like during the generation of X-ray radiation, are emitted by the accelerator and the scattering foil is in the trajectory of the electron beam although the target should be in this position, the patient is exposed to a very high electron radiation and this could be very harmful to a patient.
If the radiation is measured by the ionization chamber according to the above-noted prior art technique, there is still a certain risk that the patient receives too much radiation, because the accelerator is not switched off until after the radiation has left the accelerator and is measured and determined to be too high while already on its path to the patient. Also if the ionization chamber does not work properly there is a certain danger that the patient is exposed to excessive radiation.